Stop a nested form filling the model with its own fields

[AIC-BV]

Builds on #1529 — that PR adds the Form::$sharesModelScope discriminator this one guards with, so the diff below includes it. Happy to rebase once #1529 lands, or to fold this into it if you'd rather have one PR.

The problem

Form::setFormValues() fills the model as if the form were about to be saved, so the fields resolve against it:

$this->prepareModelsToSave($this->model, $data);

A repeater item form and a nested form are handed the parent form's model to render against (Repeater::makeItemFormWidget(), NestedForm::init()), but their fields are keys of their own data scope, not attributes of that model. Filling it from there writes foreign values onto the record whenever an item field shares its name with a model attribute — and type, name, title, status are exactly the names repeater groups tend to use.

For a plain attribute it goes unnoticed, since nothing saves the model during a refresh. For a cast attribute it fails outright:

# a model with an enum-cast `status`, and a repeater whose items have their own
status:
    type: text
items:
    type: repeater
    form:
        fields:
            status:
                type: balloon-selector
                options:
                    featured: Featured
                    plain: Plain

Refreshing the repeater (or switching/copying items, which is where we hit it) throws:

ValueError: "featured" is not a valid backing value for enum ...

The value never belonged to the model — it was only ever the item's.

The fix

if (!$this->isNested || $this->sharesModelScope) {
    $this->prepareModelsToSave($this->model, $data);
}

isNested is set in exactly three places in core: Repeater, NestedForm and FieldSet. The first two bring a data scope of their own and must not touch the model; a fieldset groups fields visually and its fields really are the model's attributes, which is what $sharesModelScope already flags. Top-level forms are unaffected, so RelationController and settings forms keep filling the model as before.

Tests

modules/backend/tests/widgets/FormNestedModelScopeTest.php puts a field named status on the model, on a fieldset, on a nested form and in a repeater group, and asserts which of the four are allowed to reach the model's attribute. The two nested cases throw the ValueError above without the guard.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Fieldset fields now correctly use the parent model’s data scope when appropriate.
  • Nested forms and repeater items preserve their own data scopes, preventing values from being written to unrelated model attributes.
  • Required-field validation now resolves attribute paths correctly across fieldsets, nested forms, and repeater items.
  • Form value population now leaves parent model attributes unchanged when values belong to an independently scoped nested form.

[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Advanced

Run ID: a970150b-6e47-418b-9923-3d905ce2b8f3

📥 Commits

Reviewing files that changed from the base of the PR and between 44e9d68 and bbe1709.

📒 Files selected for processing (4)

• modules/backend/formwidgets/FieldSet.php
• modules/backend/tests/widgets/FormFieldSetScopeTest.php
• modules/backend/tests/widgets/FormNestedModelScopeTest.php
• modules/backend/widgets/Form.php

Included review availability: Your plan provides up to 4 included reviews per hour; 3 remain after this review.

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Walkthrough

The Form widget now tracks whether a nested form shares the model scope. FieldSet propagates this value from its parent form. Model preparation skips nested forms with their own scope. Required-field checks avoid an empty attribute prefix. New tests cover fieldset, nested form, and repeater scope behavior.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: ⚪ Minimal · up to bbe17

Independent nested fields no longer populate matching parent-model attributes, while shared fieldsets retain their existing model behavior. No actionable merge risk remains.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 17.39% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 23 functions across 4 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the primary change: preventing nested form fields from filling the parent model.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

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